This invention relates to a resistive element and a method and an apparatus for producing the same, and more particularly to a resistive element which is made of a hydrogenated amorphous silicon (a-Si:H) film containing an impurity for controlling resistivity of the film and of which resistivity is maintained stable against a heat treatment and a method and an apparatus for producing such a resistive element.
In general, a hydrogenated amorphous silicon film which contains an impurity for controlling resistivity of the film has been conventionally produced by plasma chemical vapor deposition (hereinafter also referred to as "plasma CVD"), reactive sputtering or the like.
For example, formation of a hydrogenated amorphous silicon film of the n-type by plasma CVD is carried out by subjecting a starting gas material consisting of mono-silane (SiH.sub.4) or a mixture of higher silane and phosphine to radiofrequency (RF) discharge, resulting in being decomposed, followed by deposition of the decomposed gas on a substrate kept at a temperature of about 200.degree. to 300.degree. C.
The hydrogenated amorphous silicon film thus formed which contains the impurity for controlling resistivity of the film contains a hydrogen component at a level of about 10 to 20 atm %. The hydrogen component contained significantly affects properties of the hydrogenated amorphous silicon film.
Also, the hydrogen component contained in the hydrogenated amorphous silicon film not only performs a direct function of removing a dangling bond during deposition of the film but plays a part in a surface process during formation of the film and acts as a structure relaxing agent for a network. Such parts of the hydrogen component synergistically act on each other and cooperate with a thermal effect due to a temperature of the substrate, so that the above-described dangling bond may be significantly reduced.
More particularly, the hydrogenated amorphous silicon film has a Si--H bond, which acts to reduce an unstable dangling bond to provide a structural sharpness during formation of the hydrogenated amorphous silicon film and permits P (phosphor belonging to Group V of the periodic table) and B (boron belonging to Group III of the periodic table) to realize a p-n junction due to substitutional doping as in crystalline Si. Such properties of the hydrogen component in the hydrogenated amorphous silicon film is highly important in that they permit the hydrogenated amorphous silicon film to be applied to a diode, a transistor and the like.
Heating of the hydrogenated amorphous silicon film produced as described above which contains the impurity for controlling resistivity of the film causes hydrogen to generally start to be released from the film at a temperature within a range of between 250.degree. C. and 350.degree. C. Such diffusion of hydrogen indicates release of H from the Si--H Bond, resulting in the above-described dangling bond and other abnormal electron arrangement such as floating bond or the like and therefore structural defects occurring during deposition of the hydrogenated amorphous silicon film.
Thus, it will be noted that the conventional hydrogenated amorphous silicon film containing the impurity for controlling the resistivity has a disadvantage of causing a substantial variation in properties such as an increase in resistivity of the film or the like due to the structural defects due to application of heat to the film.
Also, the hydrogenated amorphous silicon film containing the impurity for controlling resistivity of the film is often used as a resistive element on a cathode conductor side of a field emission type fluorescent display device wherein a field emission cathode is used as an electron source therefor. Unfortunately, the hydrogenated amorphous silicon film, as described above, exhibits thermal instability, so that use of the film as the resistive element for the field emission type fluorescent display device causes the film to be subject to restrictions on various conditions for a heat treatment carried out during manufacturing of the device.
Thus, the hydrogenated amorphous silicon film containing the impurity for controlling resistivity of the film which is produced according to the conventional techniques fails to provide a resistive element which permits a field emission type fluorescent display device to be stably operated, while ensuring satisfactory stability and reproducibility in production of the element.